(a) Field of the Invention
The present invention relates to a material for an organic electro-optical device and an organic electro-optical device including the same. More particularly, the present invention relates to a material having thermal stability of a glass transition temperature of 120° C. or more and a thermal decomposition temperature of 450° C. or more, and being capable of providing an organic electro-optical device having high efficiency and a long life-span due to less crystallization and improved amorphous properties in a material for an organic electro-optical device, and an organic electro-optical device including the same.
(b) Background of the Invention
An electro-optical device is, in a broad sense, a device for transforming photo energy to electrical energy, and conversely, for transforming electrical energy to photo energy. The electro-optical device may be exemplified by an organic light emitting diode, a solar cell, a transistor, and so on.
Particularly, among these electro-optical devices, an organic light emitting device employing organic light emitting diodes (OLED) has recently drawn attention due to an increase in demand for flat panel displays.
The organic light emitting diode transforms electrical energy into light by applying current to an organic light emitting material. It has a structure in which a functional organic material layer is interposed between an anode and a cathode.
The organic light emitting diode has similar electrical characteristics to those of light emitting diodes (LED) in which holes are injected from an anode and electrons are injected from a cathode, then the holes and electrons move to opposite electrodes and are recombined to form excitons having high energy. The generated excitons generate light having a certain wavelengths while shifting to a ground state.
In 1987, Eastman Kodak, Inc. firstly developed an organic light emitting diode including a low molecular aromatic diamine and aluminum complex as an emission-layer-forming material (Applied Physics Letters. 51, 913, 1987). C. W Tang et al. firstly disclosed a practicable device as an organic light emitting diode in 1987 (Applied Physics Letters, 51 12, 913-915, 1987).
According to the reference, the organic layer has a structure in which a thin film (hole transport layer (HTL)) of a diamine derivative and a thin film of tris(8-hydroxy-quinolate)aluminum (Alq3) are laminated.
Generally, an organic light emitting diode is composed of an anode of a transparent electrode, an organic thin layer of a light emitting region, and a metal electrode (cathode) formed on a glass substrate, in that order. The organic thin layer may include an emission layer, a hole injection layer (HIL), a hole transport layer (HTL), an electron transport layer (ETL), or an electron injection layer (EIL). It may further include an electron inhibition layer or a hole inhibition layer due to the emission characteristics of the emission layer.
When the organic light emitting diode is applied with an electric field, the holes and electrons are injected from the anode and the cathode, respectively. The injected holes and electrons are recombined on the emission layer though the hole transport layer (HTL) and the electron transport layer (ETL) to provide light emitting excitons.
The provided light emitting excitons emit light by transiting to the ground state.
The light emission may be classified as a fluorescent material including singlet excitons and a phosphorescent material including triplet excitons according to light emitting mechanism.
Recently, it is has become known that a phosphorescent light emitting material can be used for a light emitting material of an organic light emitting diode in addition to the fluorescent light emitting material (D. F. O'Brien et al., Applied Physics Letters, 74 3, 442-444, 1999; M. A. Baldo et al., Applied Physics letters, 75 1, 4-6, 1999). Such a phosphorescent material emits lights by transiting the electrons from a ground state to an exited state, non-radiance transiting of a singlet exciton to a triplet exciton through intersystem crossing, and transiting a triplet exciton to a ground state to emit light.
When the triplet exciton is transited, it cannot directly transit to the ground state. Therefore, the electron spin is flipped, and then it is transited to the ground state so that it provides a characteristic of extending the lifetime (emission duration) to more than that of fluorescent emission.
In other words, the duration of fluorescent emission is extremely short at several nanoseconds, but the duration of phosphorescent emission is relatively long such as at several microseconds.
In addition, evaluating quantum mechanically, when holes injected from the anode are recombined with electrons injected from the cathode to provide light emitting excitons, the singlet and the triplet are produced in a ratio of 1:3, in which the triplet light emitting excitons are produced at three times the amount of the singlet light emitting excitons in the organic light emitting diode.
Accordingly, the percentage of the singlet exited state is 25% (the triplet is 75%) in the case of a fluorescent material, so it has limits in luminous efficiency. On the other hand, in the case of a phosphorescent material, it can utilize 75% of the triplet exited state and 25% of the singlet exited state, so theoretically the internal quantum efficiency can reach up to 100%. When a phosphorescent light emitting material is used, it has advantages in an increase in luminous efficiency of around four times that of the fluorescent light emitting material.
In the above-mentioned organic light emitting diode, a light emitting colorant (dopant) may be added in an emission layer (host) in order to increase the efficiency and stability in the emission state.
In this structure, the efficiency and properties of the light emission diodes are dependent on the host material in the emission layer. According to studies regarding the emission layer (host), the organic host material can be exemplified by a material including naphthalene, anthracene, phenanthrene, tetracene, pyrene, benzopyrene, chrysene, pycene, carbazole, fluorene, biphenyl, terphenyl, triphenylene oxide, dihalobi phenyl, trans-stilbene, and 1,4-diphenylbutadiene.
Generally, the host material includes 4,4-N,N-dicarbazole biphenyl (CBP) having a glass transition temperature of 110° C. or less and excessively high symmetry. Thereby, it tends to crystallize and cause problems such as a short and a pixel defect according to results of thermal resistance tests of the devices.
Recently, there have been technical developments in organic electrical light emitting devices. However, luminous efficiency, color purity, and electrical and thermal stability of the device do not approach a satisfactory level. Therefore, it is required to develop phosphorescent light emitting materials having high efficiency, high color purity, and electrical and thermal stability.